Recently, electronic devices having biometric identification function have entered people's life and work, and among them, fingerprint identification technology has drawn much attention due to unique identity property of fingerprint.
FIG. 1 is a schematic diagram illustrating principle of detecting fingerprints by using a surface structure identification circuit including photodiodes. As shown in FIG. 1, light emitted by a backlight source 200 passes through the surface structure identification circuit 100 including the photodiodes, reaches a finger surface 300 on the surface structure identification circuit 100, and is then reflected back to light receiving surfaces of the photodiodes from the finger surface 300, and the photodiodes generate currents according to the received light. Because ridges and valleys of fingerprint have different distances to the photodiodes, the current generated by the photodiode corresponding to a ridge of fingerprint is different from that generated by the photodiode corresponding to a valley of fingerprint. Therefore, fingerprint morphology can be determined based on magnitudes of the currents generated by the photodiodes in the surface structure identification circuit 100.
FIG. 2 is a circuit diagram of an existing surface structure identification circuit. As shown in FIG. 2, the surface structure identification circuit includes a plurality of scan lines 120 and a plurality of trace lines 110, which are provided in different layers and intersect with each other to define a plurality of surface structure identification units, in each of which a photoelectric sensing element D1 and a switching transistor TFT. In the process of identifying fingerprints, the plurality of scan lines 120 are scanned line by line, the photoelectric sensing element D1 is used for generating a detection current according to reflected light from the fingerprint, and then the fingerprint morphology is determined based on the magnitudes of the detection currents generated in the respective surface structure identification units.
However, when scanning one row of the surface structure identification units, a current Iout generated from a first signal input terminal and a current Idata generated by the photoelectric sensing element D1 are mixed together to be outputted, and Idata is much smaller than Iout, thus, the current generated by the photoelectric sensing element D1 can hardly be distinguished from the mixed currents, which results in an error in fingerprint identification result.
Therefore, how to recognize the current generated by the photoelectric sensing element accurately becomes an urgent technical problem to be solved in the art.